1. Field of the Invention
The present invention relates to a heat exchanger for cooling an electronic device.
2. Description of the Prior Art
Electronic devices typically generate a high concentration of heat in the power density range of 5 to 35 W/cm2. Accordingly, research activities have focused on developing more efficient cooling assemblies capable of dissipating the heat generated from such electronic devices while occupying minimal space.
A forced air cooling assembly typically includes a heat exchanger and a heat sink and cools the electronic device by natural or forced convection cooling methods. The electronic device is attached to the heat sink and transfers the heat thereto. The heat exchanger typically uses air to directly remove the heat from the heat sink. However, air has a relatively low heat capacity. Such forced air cooling assemblies are suitable for removing heat from relatively low power heat sources with a power density in the range of 5 to 15 W/cm2. However, increased computing speeds have resulted in a corresponding increase in the power density of the electronic devices in the order of 20 to 35 W/cm2, thus requiring more effective cooling assemblies.
In response, liquid-cooled cooling assemblies, commonly referred to as liquid cooled units (LCUs) were developed. The LCUs comprise a cold plate, a pump, a heat exchanger, the appropriate piping and a high heat capacity cooling fluid such as water or water-glycol solutions to remove heat from the higher power density heat sources. One type of LCU circulates the cooling fluid through the heat sink to remove the heat absorbed from the heat source affixed thereto. The cooling fluid is then transferred to a remote location where the heat is easily dissipated into a flowing air stream with the use of a liquid-to-air heat exchanger and an air moving device such as a fan or blower. These types of LCUs are characterized as indirect cooling units since they remove heat from the heat source indirectly by a secondary working fluid. Generally, a single-phase liquid removes heat from the heat sink then dissipates it into the air stream that flows through the remotely located liquid-to-air heat exchanger. Such LCUs are satisfactory for a moderate heat flux of less than 35 to 45 W/cm2. These LCUs require a pumping system to direct the flow of cooling fluid over the heat sink as well as to circulate the fluid. Typically, an overall system pressure drop of 5 psi needs to be generated by these pumping systems. 90% of this pressure drop occurs across the cold plate. Minimizing this pressure drop can dramatically improve the lifetime of the pumping system as well as reduce the number of leaks at the joints of the piping.
The LCUs of the prior art have included an axial inlet for coolant with a diverter to direct the coolant radially outward and into fins or vanes. Examples of such LCUs are illustrated in U.S. Pat. No. 4,733,293 to Gabuzda; U.S. Pat. No. 5,597,034 to Barker, III, et al.; U.S. Pat. No. 6,196,300 to Checchetti; U.S. Pat. No. 6,219,242 to Martinez and U.S. Pat. No. 6,719,038 to Bird et al. Each patent discloses a heat sink assembly having radial fins or vanes and used in an LCU. The heat sink assemblies include a base plate with a plurality of fins extending upwardly from the base plate. In operation, the base plate absorbs the heat from the electronic device and transfers the heat to the fins. A cooling fluid flows past the fins, drawing the heat from the fins, thereby removing the heat from the heat sink. The flow of cooling fluid is directed parallel to the fins by a central diverter.
A significant effort has been made in the prior art to develop the most efficient combination of parameters for attaining the maximum heat transfer with the heat sink and the minimum overall system pressure drop. Such parameters have included the configuration of the fins and the fluid flow paths associated with the fins. The U.S. Pat. No. 5,304,846 to Axar et al. is exemplary of the pursuit if the most effective combination of parameters.
Although the prior art dissipates heat from electronic devices, as computing speeds increase, there is a continuing need for cooling devices having more efficient heat transfer capabilities as well as smaller overall system pressure drops.